The cornea comprises a thin protective surface epithelium layer, typically a circular area of about 10 mm in diameter, on top of the Bowman's membrane or layer, which in turn covers the major corneal stroma. Although the epithelium has no blood cells it does have nerve cell endings. When the corneal epithelium is eroded, cut, damaged, dystrophied or diseased, it can be removed and it will regenerate in about 2-3 days. The epithelium is also removed as a precursor to laser refractive surgery of using a laser over a circular area of a diameter somewhat larger than 6 mm.
While the epithelium is regenerative, the underlying Bowman's membrane is not. With ablative corneal tissue removal procedures, such as PRK (photo-refractive keratectomy, an ablative procedure using an excimer laser-based system), the epithelium and Bowman's membrane are removed together with a portion of the stroma. As soon as the epithelium is removed as the first step in the PRK procedure the cornea at first dehydrates and the begins to hydrate which changes the ablation rate for PRK and the thickness of the cornea and the amount of material actually removed become uncertain. In addition, the longer the time period for epithelium removal, the greater the uncertainty with respect to the degree of hydration and the greater the inaccuracy of the PRK. In addition, debris may locally block or mask photo ablation with formation of isolated central islands of unremoved materials.
After removal, the epithelium regenerates on the exposed outer surface of the cornea either on the Bowman's layer or directly on the stroma, if the Bowman's layer has been removed, since the Bowman's layer is not regenerated. Direct regrowth of the epithelium on the stroma can however cause an undesirable corneal haze which gradually dissipates over time.
Removal of the epithelium alone, if necessary, is usually generally accomplished by means of scraping with a surgical blade. However, this is a rough, imperfect, and inaccurate means for removal of the epithelium and its use tends to damage the underlying Bowman's layer. Blade scraping is also slow and tedious and requires about a minute or more to remove the epithelium. Because of the scraping, the new surface of the anterior surface of the Bowman's layer, has substantial debris and is usually badly damaged. Evidence of such damage is readily ascertained by viewing under high magnification scanning electron microscope which shows that the new exposed surface does not exhibit the typical honeycomb appearance of the Bowman's layer.
Solvent removal of the epithelium is the most efficacious method for removal, without physical damage to the Bowman's layer and over a well defined area. Thus, a solvent such as alcohol is capable of cleanly dissolving away the epithelium. However, alcohol and other similar solvents are toxic and cannot be safely used on live eyes.
A new procedure has been developed for removal of the epithelium layer wherein a circular, rotating brush is pressed against the epithelium to quickly wear the epithelium away. The extent of damage this causes to the Bowman's layer is similar to that of the blade and it appears that the procedure is operator dependent with the amount of damage to the Bowman's layer being related to the degree of engagement pressure between brush and cornea as exerted by the operator.
The PRK system is also capable of removing epithelium. However, the epithelium is generally not of uniform thickness and it is not possible to remove all of the epithelium without removing some of the Bowman's layer.